Looking at http://people.redhat.com/~katzj/anaconda-farsi.png it seems
like the anaconda UI wasn't properly mirrored, even though the text is
RTL. 

What happens wrt to RTL mirroring in gtk+ is:
At init (in gtk_parse_args) the default direction is set based on the
current language, like this:

    char *e = _("default:LTR");
    if (strcmp (e, "default:RTL")==0) {
      gtk_widget_set_default_direction (GTK_TEXT_DIR_RTL);
    } else if (strcmp (e, "default:LTR")) {
      g_warning ("Whoever translated default:LTR did so wrongly.\n");
    }

If you change the locale at runtime, after initializing gtk you need
to call gtk_widget_set_default_direction with the correct flags. You
can probably look at dgettext("gtk20", "default:LTR") after you've
changed the language, and use code similar to above to set the direction.